楼盖和屋盖采用钢筋混凝土结构，而墙和柱采用砌体结构建造的房屋属于()体系建筑。

A.混合结构 

B.框架结构 

C.剪力墙 

D.桁架结构 

Only individual benefactors and ad hoc grants have made possible the ecological surveys already undertaken.

A．additional

B．governmental

C．special

D．organizational